Field effect device with reduced thickness gate

ABSTRACT

A semiconductor structure is fabricated with reduced gate capacitance by thinning of a gate electrode to provide a reduced thickness gate electrode. The gate electrode is thinned after forming a spacer layer adjoining the gate electrode. In addition, the height of the spacer layer may also be reduced. The spacer layer thus has an enhanced horizontal width desired for locating an intrinsic source/drain with respect to an extension region and in particular, an enhanced horizontal width relative to the spacer height. The reduced thickness gate electrode may be fully silicided to provide decreased gate resistance. A raised source/drain layer may be located upon the intrinsic source/drain region. The raised source/drain layer may have a top surface higher than the reduced thickness gate electrode. In addition, the raised source/drain layer may have a top surface higher than the reduced height spacer layer.

BACKGROUND

1. Field of the Invention

The invention relates generally to semiconductor devices such as fieldeffect transistor (FET) devices. More particularly, the inventionrelates to efficient methods for fabricating semiconductor devices, suchas field effect transistor devices with enhanced performance.

2. Description of the Related Art

Integrated circuits commonly comprise field effect transistor devices asactive switching elements. For purposes of reducing power consumption,field effect transistor devices are typically provided as complementarydoped pairs. Various factors affect field effect transistor deviceperformance. Non-limiting examples of such factors include dimensional,materials of composition, mechanical stress effect and doping relatedfactors.

Attention has recently focused upon the gate electrode compositionrelated performance effects when fabricating field effect transistordevices. To that end, field effect transistor devices fabricated withfully silicided gate electrodes are desirable since fully silicided gateelectrodes often have enhanced conductivity, and they are not subject topolysilicon dopant depletion phenomena. Fully silicided gate electrodesalso have other performance advantages in comparison with other gateelectrode materials.

Various field effect transistor structures using suicide gateelectrodes, and methods for fabrication thereof, are known in the art.

For example, Xiang et al., in U.S. Pat. No. 6,562,718, teaches a methodfor fabricating a field effect transistor structure with a fullysilicided gate electrode. The method disclosed in the '718 patent uses ashielding layer located upon a pair of silicided source/drain regions,but not a partially silicided gate electrode within the field effecttransistor structure, so that the partially silicided gate electrode maybe fully silicided without affecting the silicided source/drain regions.

In addition, Gong et al., in U.S. Pat. No. 6,902,994, teaches anadditional method for fabricating a field effect transistor structurewith a fully silicided gate electrode. The method disclosed in the '994patent provides for simultaneous silicidation of a pair of raisedsource/drain regions and a silicon gate electrode after removing acapping layer used to protect the silicon gate electrode when formingthe pair of raised source/drain regions.

Further, Lin et al., in U.S. Pat. No. 6,905,922, teaches a method forfabricating dual fully silicided gate electrodes within field effecttransistor structures. This prior method uses a protective layer formedupon a pair of silicided source/drain regions when fully siliciding thegate electrodes.

Still further, Wen et al., in U.S. Patent Application Publication2005/0156238, also teaches a silicided gate field effect transistorstructure and a method for fabricating the same. The Wen et al. methodprovides for protecting a pair of silicided source/drain regions whensiliciding a silicon gate electrode within the field effect transistordevice.

Finally, Bu et al., in U.S. Patent Application Publication 2005/0215055,also teaches a silicided gate field effect transistor structure and amethod for fabricating the same. The Bu et al. method provides forforming a silicided gate electrode prior to a pair of sillicidedsource/drain regions.

As semiconductor device technology continues to advance, needs continueto exist for fabricating field effect transistor devices with enhancedperformance. To that end, desirable are additional field effecttransistor structures having silicided gate electrodes, as well asmethods for fabrication thereof.

SUMMARY OF THE INVENTION

The invention provides a semiconductor structure, in particularly afield effect transistor structure, and methods for fabricating the same.

A semiconductor structure in accordance with the invention comprises asemiconductor substrate having a channel region separating a pair ofsource/drain regions. The structure also includes a gate electrodelocated over the channel region. Within the structure, a top surface ofthe gate electrode is no higher than a top surface of the pair ofsource/drain regions.

One method for fabricating the inventive structure includes forming anextension region into a semiconductor substrate which includes at leasta gate electrode of a first thickness thereon. The gate electrode servesas a mask when forming the extension region. The method also includesforming an intrinsic source/drain region into the semiconductorsubstrate while using the gate electrode and a horizontal spacer layerformed adjacent thereto as a mask. Finally, the method also includesetching the gate electrode to a second thickness which is less than thefirst thickness, thereby forming a reduced thickness gate electrode.

Another method for fabricating a semiconductor structure in accordancewith the invention includes forming an extension region into asemiconductor substrate which includes at least a gate electrode stackthereon. The gate electrode stack serves as a mask when forming theextension region. The gate electrode stack includes a reduced thicknessgate electrode, an etch stop layer thereupon and a vertical spacer layerfurther thereupon. The method also includes forming an intrinsicsource/drain region into the semiconductor substrate while using thegate electrode stack and a horizontal spacer layer formed adjacentthereto as a mask. Finally, the method includes stripping the verticalspacer layer from the gate electrode stack while using the etch stoplayer as a stop layer, and then further stripping the etch stop layer toexpose the reduced thickness gate electrode.

Within the foregoing structure, and methods of forming the same, thegate electrode or reduced thickness gate electrode may be fullysilicided.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the invention are understoodwithin the context of the Description of the Preferred Embodiment, asset forth below. The Description of the Preferred Embodiment isunderstood within the context of the accompanying drawings, that form amaterial part of this disclosure, wherein:

FIG. 1 to FIG. 8 show a series of schematic cross-sectional diagramsillustrating the results of progressive stages in fabricating a fieldeffect transistor device in accordance with a first embodiment of theinvention.

FIG. 9 to FIG. 17 show a series of schematic cross-sectional diagramsillustrating the results of progressive stages in fabricating a fieldeffect transistor device in accordance with a second embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention, which provides a semiconductor structure having areduced gate thickness and methods for fabricating the same, will now bedescribed in greater detail.

Reference is first made to FIGS. 1-8 which show a series of schematiccross-sectional diagrams illustrating the results of progressive stagesin fabricating a field effect transistor device in accordance with afirst embodiment of the invention.

FIG. 1 shows a semiconductor substrate 10 having a pair of extensionregions 12 a and 12 b located within the semiconductor substrate 10. Thepair of extension regions 12 a and 12 b is separated by a channel regionwithin the semiconductor substrate 10. Located above the channel regionis a gate electrode stack 13. The gate electrode stack 13 comprises, inprogressively layered sequence upon the channel region within thesemiconductor substrate 10: (1) a gate dielectric 14; (2) a silicon gateelectrode 16 located upon the gate dielectric 14; and (3) a cappinglayer 22 located upon the silicon gate electrode 16.

Each of the foregoing semiconductor substrate 10, layers and structuresmay comprise materials, have dimensions and be formed using methods thatare otherwise conventional in the semiconductor fabrication art.

The semiconductor substrate 10 may comprise semiconductor materialsincluding, but not limited to: silicon, germanium, silicon-germaniumalloy, silicon carbide, silicon-germanium carbide alloy, or compoundsemiconductor materials. Non-limiting examples of compound semiconductormaterials include gallium arsenide, indium arsenide and indium phosphidesemiconductor materials. The semiconductor substrate 10 may comprise abulk semiconductor substrate (as is generally illustrated in FIG. 1) orit may alternatively comprise a semiconductor-on-insulator substrate (asmight more specifically be illustrated within FIG. 1 when thesemiconductor substrate 10 additionally comprises and illustrates aburied dielectric layer that separates a lower base semiconductorsubstrate and a surface semiconductor layer within the semiconductorsubstrate 10). The semiconductor substrate 10 may also comprise a hybridorientation technology semiconductor substrate that further comprisesmultiple regions with different crystallographic orientations.

The pair of extension regions 12 a and 12 b comprise either N or Ppolarity dopants that are appropriate for the polarity of the fieldeffect transistor desired to be fabricated in accordance with the firstembodiment. The pair of extension regions 12 a and 12 b is formed intothe semiconductor substrate 10 while using the gate electrode stack 13(and in particular the silicon gate electrode 16) as a mask, and whileusing an ion implant method and materials that are otherwise generallyconventional in the semiconductor fabrication art. The pair of extensionregions 12 a and 12 b will typically have either an N or a P polarity.Each of the pair of extension regions 12 a and 12 b is typically formedusing a dopant dose from about 1e15 to about 2e15 dopant atoms persquare centimeter.

The gate dielectric 14 may comprise generally conventional dielectricmaterials, such as silicon oxides, silicon nitrides and siliconoxynitrides having a dielectric constant from about 4 to about 20,measured in a vacuum. Alternatively, the gate dielectric 14 may comprisegenerally higher dielectric constant dielectric materials having adielectric constant from 20 to at least about 100. Such higherdielectric constant dielectric materials may include, but are notlimited to: hafnium oxides, hafnium silicates, titanium oxides, rareearth oxides, barium-strontium titanates (BSTs) and lead-zirconatetitanates (PZTs). The dielectric materials may be formed using methodsincluding, but not limited to: thermal or plasma deposition or annealingmethods, chemical vapor deposition methods, atomic layer depositionmethods and physical vapor deposition methods. Typically, the gatedielectric 14 comprises a thermal silicon oxide material having athickness from about 10 to about 70 angstroms.

The silicon gate electrode 16 may comprise any of several siliconmaterials, as well as silicon-germanium alloy materials with germaniumconcentrations from about zero to about 100 atomic percent germanium.Alternatively, the silicon gate electrode 16 may comprise a laminatecomprising germanium followed by any of several silicon materials. Thesilicon or silicon-germanium alloy materials may include, but are notlimited to: polycrystalline materials and amorphous materials.Polycrystalline silicon and silicon-germanium alloy materials are mostcommon. The silicon gate electrode 16 may be deposited using methodsthat are conventional in the art. The methods may include, but are notlimited to: chemical vapor deposition methods and physical vapordeposition methods. Typically, the silicon gate electrode 16 comprises apolycrystalline silicon material that has a thickness from about 700 toabout 1000 angstroms.

The capping layer 22 may comprise any of several capping materials.Typically, the capping layer 22 comprises a dielectric capping material.Silicon oxide, silicon nitride and silicon oxynitride dielectric cappingmaterials are common. A silicon nitride capping material is oftendesirable and thus common. The capping layer 22 may be deposited usinggenerally conventional methods that are used for depositing the gatedielectric 14 and the silicon gate electrode 16. Typically, the cappinglayer 22 has a thickness from about 200 to about 400 angstroms.

In order to fabricate the semiconductor structure that is illustrated inFIG. 1, the semiconductor substrate 10 is first provided. In sequence, ablanket gate dielectric layer, a blanket silicon layer and a blanketcapping layer are formed upon or over the semiconductor substrate 10.The foregoing blanket layers are then sequentially patterned while usinga patterned photoresist layer (not shown) as an etch mask layer, toprovide the gate electrode stack 13 shown in FIG. 1. The gate electrodestack 13 is then used as an ion implantation mask for forming the pairof extension regions 12 a and 12 b within the semiconductor substrate10.

FIG. 2 shows a pair of horizontal spacer layers 24 a and 24 b locatedadjacent and adjoining a pair of opposite sidewalls of the gatedielectric 14, the gate electrode 16 and the capping layer 22, to thusprovide a spacer clad gate electrode stack 13′. It should be appreciatedthat the pair of horizontal spacer layers 24 a and 24 b is for thecross-section illustrated. In a top down view, the pair of horizontalspacers form a single spacer that is present around the gate stackregion.

The pair of horizontal spacer layers 24 a and 24 b may comprise a spacermaterial including, but not limited to: conductor spacer materials,semiconductor spacer materials and dielectric spacer materials.Dielectric spacer materials are most common, but conductor spacermaterials are also known. Also known are laminates of dielectric spacermaterials and conductor spacer materials. Typically, the pair of spacerlayers 24 a and 24 b is formed using a blanket spacer material layerdeposition and anisotropic etchback method. The blanket spacer materiallayer may be deposited using any of several methods. Non-limitingexamples include thermal or plasma oxidation or nitridation methods,chemical vapor deposition methods and physical vapor deposition methods.

FIG. 2 also shows a pair of intrinsic source/drain regions 12 a′ and 12b′ located within the semiconductor substrate 10 and incorporating thepair of extension regions 12 a and 12 b. The pair of intrinsicsource/drain regions 12 a′ and 12 b′ is formed using a second ionimplantation method, while using the spacer clad gate electrode stack13′ (including in particular the silicon gate electrode 16) as a mask.Implanted are ions of the same polarity that are used when forming thepair of extension regions 12 a and 12 b that are illustrated in FIG. 1.Typically, the pair of intrinsic source/drain regions 12 a′ and 12 b′ isimplanted to provide a concentration from about 1e18 to about 1e22dopant atoms per cubic centimeter. Under certain circumstances, (e.g.,particularly with raised source/drain regions as discussed below)junction depths for the pair of intrinsic source/drain regions 12 a′ and12 b′ need not necessarily be deeper than junction depths for the pairof extension regions 12 a and 12 b.

As is understood by a person skilled in the art, a horizontal dimensionof the pair of horizontal spacer layers 24 a and 24 b governs ahorizontal offset and extension of the pair of extension regions 12 aand 12 b that is incorporated into the pair of intrinsic source/drainregions 12 a′ and 12 b′. In turn, when the pair of horizontal spacerlayers 24 a and 24 b is formed using a blanket layer deposition andanisotropic etchback method, the horizontal dimension of the pair ofhorizontal spacer layers 24 a and 24 b is governed by the height of thegate electrode stack 13 that includes the gate electrode 16. Thus, inorder to provide a wider horizontal offset of the pair of extensionregions 12 a and 12 b with respect to the pair of intrinsic source/drainregions 12 a′ and 12 b′, the gate electrode stack 13 that is illustratedin FIG. 1 must be higher.

FIG. 3 shows a pair of raised source/drain layers 26 a and 26 b that arelocated upon the pair of intrinsic source/drain regions 12 a′ and 12 b′.Within the context of the instant embodiment and the invention either:(1) the pair of intrinsic source/drain regions 12 a′ and 12 b′themselves; or (2) the laminated pair comprising intrinsic source/drainregions 12 a′ and 12 b′ laminated with raised source/drain layers 26 aand 26 b, is intended as a pair of “source/drain regions.” FIG. 3 alsoshows a pair of protective layers 28 a and 28 b located upon the pair ofraised source/drain layers 26 a and 26 b.

The pair of raised source/drain layers 26 a and 26 b typically has thesame crystallographic orientation as the pair of intrinsic source/drainregions 12 a′ and 12 b′. The pair of raised source/drain layers 26 a and26 b is typically formed using an epitaxial chemical vapor depositionmethod. This method may provide for intrinsic doping of the pair ofraised source/drain layers 26 a and 26 b (i.e., of same polarity as thepair of intrinsic source/drain regions 12 a′ and 12 b′). Alternatively,the pair of raised source/drain layers 26 a and 26 b may be subsequentlydoped using methods such as, but not limited to: ion implantationmethods and thermal diffusion methods.

In addition, while the pair of raised source/drain layers 26 a and 26 btypically comprises the same semiconductor material as the pair ofintrinsic source/drain regions 12 a′ and 12 b′, neither the embodimentnor the invention is so limited. Rather, the pair of raised source/drainlayers 26 a and 26 b may comprise a different semiconductor material,particularly under circumstances where the pair of raised source/drainlayers 26 a and 26 b may assist in providing a mechanical stress orstrain into the channel region of the field effect transistor whoseschematic cross-sectional diagram is illustrated in FIG. 3. Typically,each of the pair of raised source/drain layers 26 a and 26 b has athickness from about 200 to about 1000 angstroms.

The pair of protective layers 28 a and 28 b may comprise any of severalprotective materials including, but not limited to: conductor protectivematerials and dielectric protective materials. The pair of protectivelayers 28 a and 28 b is intended as a pair of silicidation protectivelayers with respect to the pair of raised source/drain layers 26 a and26 b. The pair of protective layers 28 a and 28 b may be formed usingany of several methods. Non-limiting examples include thermal or plasmaoxidation methods or nitridation methods, chemical vapor depositionmethods and physical vapor deposition methods. As is illustrated in FIG.3, the pair of protective layers 28 a and 28 b is preferably of aconformal shape with respect to the pair of raised source/drain layers26 a and 26 b. As may be implied by the specific shape of each of theprotective layers 28 a and 28 b, they are preferably formed selectively.Preferably, a thermal oxidation method is used to provide the pair ofprotective layers 28 a and 28 b comprising a thermal silicon oxidematerial having a thickness from about 100 to about 300 angstroms.During formation of the protective layers 28 a and 28 b, capping layer22 protects the top of silicon gate electrode 16 from oxidation.

FIG. 4 first shows the results of stripping the capping layer 22 fromthe spacer clad gate electrode stack 13′ that is illustrated in FIG. 2and FIG. 3. FIG. 4 also shows the results of etching the silicon gateelectrode 16 to form a reduced thickness silicon gate electrode 16′.

The capping layer 22 may be stripped using methods and materials thatare appropriate to its materials of composition. The methods andmaterials may include, but are not limited to: wet chemical methods andmaterials, dry plasma methods and materials and aggregate methods andmaterials thereof. When the capping layer 22 comprises a preferredsilicon nitride material, the capping layer 22 may be strippedselectively with respect to the pair of spacer layers 24 a and 24 b, thepair of protective layers 28 a and 28 b, and the silicon gate electrode16, while using either an aqueous phosphoric acid etchant at an elevatedtemperature (typically from about 80° C. to about 90° C., but possiblyalso at a higher temperature), or alternatively a plasma etch methodhaving an etchant gas composition engineered for the foregoing etchspecificity.

Alternatively, if the capping layer 22 and the spacer layers 24 a and 24b are all formed of the same material (e.g., a silicon nitridematerial), then a single etch step (such as, but not limited to: a dryplasma etch step or a wet chemical etch step) could simultaneouslyexpose the top of the gate electrode 16 and reduce the height of thespacer layers 24 a and 24 b. If such etching is continued, a height ofthe spacer layers may be recessed to the level denoted by dotted linesillustrated in FIG. 4, thus providing reduced height spacer layers 24 cand 24 d. Alternatively, the same spacer layer recess could also beaccomplished by separate etching processing steps when the capping layer22 and the spacer layers 24 a and 24 b are comprised of materials withdiffering etch selectivities.

The gate electrode 16 may be etched to form the reduced thickness gateelectrode 16′ while also using an appropriate etchant that may be eithera wet chemical etchant or an appropriate dry plasma etchant. Typically,the gate electrode 16 is etched to form the reduced thickness gateelectrode 16′ while using an anisotropic plasma etch method that uses achlorine containing etchant gas composition. In accord with disclosureabove, the gate electrode 16 preferably has a thickness from about 700to about 1000 angstroms. The gate electrode 16 is etched to form thereduced thickness gate electrode 16′ that preferably has a thicknessfrom about 50 to about 500 angstroms.

In a particularly advantageous embodiment, the capping layer 22 iscomprised of nitride and is removed by an anisotropic nitride plasmaetch which is selective to a silicon dioxide protective layer 28 a and28 b, then the etch chemistry is switched to a second etch chemistrywhich etches the silicon gate electrode 16 as described above. Thesecond etch chemistry etches the silicon gate electrode 16 selectivelyto the silicon dioxide protective layers 28 a and 28 b. The first etchchemistry which etches nitride may comprise a CH₃F/O₂/He mixture in apreferred ratio from about 1:3:4 to about 1:1:2, at a total pressurefrom about 20 to about 40 mTorr and at a plasma power from about 400 Wto about 500 W. The second etch chemistry which etches silicon maycomprise a chlorine containing plasma. SF₆/Cl₂ mixtures at a ratio fromabout 1:1 to about 4:1 using a total pressure from about 10 to about 50mTorr and plasma powers from about 200 to about 400 W are useful whencoupled with a low bias power below 100 W.

In a first instance, the schematic cross-sectional diagram of FIG. 4illustrates a field effect transistor in part in accordance with a firstembodiment of the invention. The field effect transistor has a reducedheight silicon gate electrode 16′ in comparison with the silicon gateelectrode 16 that is illustrated in FIG. 1. As disclosed above, athickness of the gate electrode stack 13 (including the silicon gateelectrode 16) determines, in part, a horizontal dimension of a pair ofhorizontal spacer layers 24 a and 24 b that is used to determine ahorizontal extension dimension of a pair of extension regions 12 a and12 b incorporated within a pair of intrinsic source/drain regions 12 a′and 12 b′. However, for gate electrode capacitance reduction, a thinnergate electrode is desirable. Thus, the first embodiment: (1) uses athicker silicon gate electrode 16 within a gate electrode stack 13 forpurposes of forming a desirably comparatively wider sized pair of spacerlayers 24 a and 24 b; and (2) subsequently thins the silicon gateelectrode 16 to provide a reduced thickness silicon gate electrode 16′that provides reduced gate capacitance. The foregoing aspects of thepresent invention are realized independent of the material ofcomposition of the spacer layers 24 a and 24 b and the silicon gateelectrode 16 (i.e., other than a silicon gate electrode may be used).The foregoing aspects of the invention are also realized within thecontext of a pair of raised source/drain layers 26 a and 26 b. The pairof raised source/drain layers 26 a and 26 b is of sufficient height thatthe reduced thickness silicon gate electrode 16′ may be not higher than,or alternatively lower than, the reduced thickness silicon gateelectrode 16′. Also, spacer layers 24 a and 24 b may optionally bereduced in height by an appropriate etching step to a height as denotedby dotted line that is representative of reduced height spacer layers 24c and 24 d. The final height of the reduced height spacer layers 24 cand 24 d may be below the height of the raised source/drain layers 26 aand 26 b.

The foregoing: (1) spacer layer width and extension region placementaspects of the first embodiment; and (2) reduced thickness silicon gateelectrode capacitance reduction aspects of the first embodiment,comprise initial aspects of the first embodiment. An additional aspectof the first embodiment is the height differential between the raisedsource/drain layers 26 a and 26 b that comprise “source/drain regions,”and the reduced thickness silicon gate electrode 16′. An additionalaspect of the first embodiment is the height differential between theraised source/drain layers 26 a and 26 b that comprise “source/drainregions,” and the optionally reduced height spacer layers 24 c and 24 d.The foregoing aspects do not, however, complete the first embodiment.Rather, a further aspect of the first embodiment is illustrated incidentto further processing of the semiconductor structure whose schematiccross-sectional diagram is illustrated in FIG. 4. The further processingis illustrated within the schematic cross-sectional diagrams of FIG. 5to FIG. 8.

FIG. 5 shows a metal suicide forming metal layer 30 located upon thesemiconductor structure whose schematic cross-sectional diagram isillustrated in FIG. 4.

The metal silicide forming metal layer 30 may comprise any of severalmetal silicide forming metals. Non-limiting examples include titanium,tungsten, cobalt, nickel, vanadium and platinum metal silicide formingmetals. Also included are alloys including the foregoing metal silicideforming metals. The metal silicide forming metal layer 30 may bedeposited using any of several methods. Non-limiting examples includeevaporative and other physical vapor deposition methods, as well aschemical vapor deposition methods. Preferably, the metal silicideforming metal layer 30 has a thickness sufficient to completely consumethe reduced thickness silicon gate electrode 16′ when the reducedthickness silicon gate electrode 16′ is thermally annealed in contactwith the metal silicide forming metal layer 30. Typically, the metalsilicide forming metal layer 30 has a thickness from about 20 to about3000 angstroms in order to effect the foregoing full consumption of thereduced thickness silicon gate electrode 16′. If present, the pair ofreduced height spacer layers 24 c and 24 d also reduces the aspect ratioof a pair of gaps adjoining thereto to be partially filled duringdeposition of the metal silicide forming metal layer 30. In addition,the presence of the pair of reduced height spacer layers 24 c and 24 dalso reduces fringing capacitance of the reduced thickness silicon gateelectrode 16′, or a silicide gate electrode formed therefrom.

FIG. 6 shows a silicide gate electrode 15 that results from thermallyannealing the reduced thickness silicon gate electrode 16′ in contactwith the metal silicide forming metal layer 30 that is illustrated inFIG. 5. FIG. 6 also shows a partially consumed metal silicide formingmetal layer 30′ that results from partial consumption of the metalsilicide forming metal layer 30 when forming the silicide gate electrode15. To form the silicide gate electrode 15, the semiconductor structureof FIG. 5 is typically thermally annealed for a time period from about30 seconds to about 30 minutes, at a temperature from about 300° C. toabout 800° C. Alternatively operative thermal annealing methods andconditions may also be used.

Formation of certain silicide phases may fully consume the metalsilicide forming metal layer 30. Particular silicide phases aredetermined by metal selection, amount of metal deposition for a giventhickness of silicide gate electrode 15, and anneal temperature. Also,when at least a portion of the reduced thickness silicon gate electode16′ includes a germanium material, then a metal germanide is formed ifthe metal silicide forming metal layer 30 is also susceptible to forminga metal germanide.

FIG. 7 shows the results of stripping the partially consumed metalsilicide forming metal layer 30′, and the pair of protective layers 28 aand 28 b, from the semiconductor structure of FIG. 6. The partiallyconsumed metal silicide forming metal layer 30′ may be stripped usingmethods and materials that are appropriate to its composition. Wetchemical etch methods, dry plasma etch methods and aggregate methodsthereof are included as non-limiting examples. Aqueous acidic wetchemical etch methods are particularly common. Specific acidcompositions and mixtures typically have a specificity with respect tospecific metal silicide forming metals. Similarly, the pair ofprotective layers 28 a and 28 b may be stripped using methods andmaterials that are specific to their materials of composition. Wetchemical methods, dry plasma methods and aggregate methods thereof arealso included as non-limiting methods. When the pair of protectivelayers 28 a and 28 b comprises a silicon oxide material, they may bestripped using either an aqueous hydrofluoric acid etchant, oralternatively a plasma etch method using a fluorine containing etchantgas composition.

FIG. 8 shows a pair of silicide layers 32 a and 32 b located upon a pairof partially consumed raised source/drain layers 26 a′ and 26 b′.

The pair of silicide layers 32 a and 32 b may comprise silicidematerials analogous, equivalent or identical to the silicide materialsthat comprise the silicide gate electrode 15. The pair of silicidelayers 32 a and 32 b may be formed using an analogous self-alignedsilicide (i.e., salicide) method (i.e., blanket layer deposition,thermal annealing and excess metal strip) that is used for forming thesilicide gate electrode 15. The pair of silicide layers 32 a and 32 bpreferably has a thickness from about 50 to about 200 angstroms. Thus,complete consumption of the pair of partially consumed raisedsource/drain layers 26 a′ and 26 b′ is not intended when forming thepair of silicide layers 32 a and 32 b.

Finally, within the first embodiment, the pair of silicide layers 32 aand 32 b located upon the pair of partially consumed raised source/drainlayers 26 a′ and 26 b′ is formed after silicidation of the reducedheight silicon gate electrode 16′ when forming the silicide gateelectrode 15.

FIG. 8 illustrates a schematic cross-sectional diagram of a field effecttransistor in accordance with the first embodiment of the invention. Thefield effect transistor has: (1) a desirable comparatively wideextension region offset within a pair of intrinsic source/drain regions12 a′ and 12 b′, due to a comparatively wide pair of horizontal spacerlayers 24 a and 24 b; (2) a desirable comparatively low gate electrodecapacitance due to thinning of a silicon gate electrode 16 to form areduced thickness silicon gate electrode 16′; (3) a desirablecomparatively low gate electrode resistance by reacting the reducedthickness silicon gate electrode 16′ with a metal silicide forming metallayer 30 to form a silicide gate electrode 15; and (4) a generallyinverted topography where the reduced thickness silicon gate electrode16′ or silicide gate electrode 15 has a thickness no higher than, oralternatively lower than, a pair of raised source/drain layers 26 a and26 b that comprise a pair of “source/drain regions.” Optionally, a pairof reduced height spacer layers 24 c and 24 d may also be included tolimit gate electrode fringing capacitance.

FIG. 9 to FIG. 17 show a series of schematic cross-sectional diagramsillustrating the results of progressive stages in fabricating a fieldeffect transistor in accordance with a second embodiment of theinvention.

The second embodiment of the invention that is illustrated in FIG. 9 toFIG. 17 correlates generally with the first embodiment of the inventionthat is illustrated in FIG. 1 to FIG. 8. However, as an exception, thegate electrode stack 13″ that is illustrated in FIG. 9 (in comparisonwith the gate electrode stack 13 that is illustrated in FIG. 1)comprises: (1) the gate dielectric 14; (2) the reduced thickness silicongate electrode 16′ located upon the gate dielectric 14; (3) an etch stoplayer 18 located upon the reduced thickness silicon gate electrode 16′;(4) a vertical spacer layer 16″ located upon the etch stop layer 18; and(5) the capping layer 22 located upon the vertical spacer layer 16″.

Of the foregoing layers within the gate electrode stack 13″, the gatedielectric 14, the reduced thickness silicon gate electrode 16′ and thecapping layer 22 are analogous equivalent or identical to the samedesignated layers within the first embodiment. Attention is drawn to thenewly introduced etch stop layer 18 and vertical spacer layer 16″.

The etch stop layer 18 is intended to provide etch stop properties withrespect to etching and removal of the vertical spacer layer 16″. Theetch stop layer 18 may thus comprise etch stop materials that areconventional in the semiconductor fabrication art. Non-limiting examplesinclude conductor etch stop materials, semiconductor etch stop materialsand dielectric etch stop materials. Most common are dielectric etch stopmaterials, and more particularly common are silicon oxide or siliconnitride dielectric etch stop materials. The etch stop materials may beformed using any of several methods. Included are thermal or plasmaoxidation or nitridation methods, chemical vapor deposition methods andphysical vapor deposition methods. Preferably, the etch stop layer 18comprises a silicon nitride or thermal silicon oxide material that has athickness from about 20 to about 70 angstroms.

The vertical spacer layer 16″ may analogously comprise any of severalspacer materials. Non-limiting examples include conductor spacermaterials, semiconductor spacer materials and dielectric spacermaterials. Preferably, within the second embodiment, the vertical spacerlayer 16″ comprises a silicon spacer material that has a compositionanalogous, equivalent or identical to the composition of the reducedthickness silicon gate electrode 16′. Such a silicon spacer material maybe deposited using an identical method and materials to those used fordepositing the reduced height silicon gate electrode 16′, with anadditional intervening thermal oxidation process step. Within the secondembodiment, the etch stop layer 18 and the vertical spacer layer 16″will together typically have a thickness from about 500 to about 950angstroms.

FIG. 10 correlates with FIG. 2, with the exception of the foregoingstructural differences in the gate electrode stack 13 (FIG. 1) and thegate electrode stack 13″ (FIG. 9), in turn, yielding analogousdifferences between the spacer clad gate electrode stack 13′ (FIG. 2)and a spacer clad gate electrode stack 13′″ (FIG. 10).

FIG. 11 correlates with FIG. 3 and illustrates analogous or equivalentpresence of the pair of raised source/drain layers 26 a and 26 b andprotective layers 28 a and 28 b. Within the second embodiment, the pairof protective layers 28 a and 28 b may desirably have an increasedthickness in comparison with the etch stop layer 18, under circumstanceswhere the etch stop layer 18 and the pair of protective layers 28 a and28 b comprise the same material, such as a preferred silicon oxidematerial.

FIG. 12 shows the results of stripping the capping layer 22 and thevertical spacer layer 16″ from the semiconductor structure whoseschematic cross-sectional diagram is illustrated in FIG. 11. Methods andmaterials for stripping the capping layer 22 are discussed in greaterdetail above within the context of the first embodiment. Methods andmaterials for stripping the spacer layer 16″ when the spacer layer 16″is formed of a silicon material, are analogous, equivalent or identicalto the methods for etching the silicon gate electrode 16 to yield thereduced thickness silicon gate electrode 16′ in accord with the firstembodiment. Also, in accordance with the first embodiment, the spacerlayers 24 a and 24 b can be optionally reduced in height by etching asdenoted by dashed lines 24 c and 24 d. The etching of the spacer layers24 a and 24 b to from the reduced height spacer layers 24 c and 24 d maybe undertaken before or after exposing the reduced thickness silicongate electrode 16′.

Primarily, the second embodiment thus differs from the first embodimentby the incorporation and use of an etch stop layer 18 that provides forgreater process control when eventually exposing a reduced thicknesssilicon gate electrode 16′.

FIG. 13 shows the results of stripping the etch stop layer 18 to leaveexposed the reduced thickness silicon gate electrode 16′. In accordancewith disclosure above, when the etch stop layer 18 and the pair ofprotective layers 28 a and 28 b are formed the same material, such as asilicon oxide material, the pair of protective layers 28 a and 28 b(which are desirably of greater thickness than the etch stop layer 18)is also simultaneously etched to form a pair of reduced thicknessprotective layers 28 a′ and 28 b′. Such an etching is not a limitationof the second embodiment, since the etch stop layer 18 and the pair ofprotective layers 28 a and 28 b need not necessarily comprise anidentical chemical composition. For example, if the etch stop layer 18is formed of silicon nitride, it can be readily selectively etched by adry plasma etchant or wet chemical etchant relative to a pair ofprotective layers 28 a and 28 b formed of silicon oxide.

FIG. 14 and FIG. 15 correlate with FIG. 5 and FIG. 6, but with thepresence of the pair of reduced thickness protective layers 28 a′ and 28b′ (FIG. 14 and FIG. 15) in comparison with the protective layers 28 aand 28 b (FIG. 5 and FIG. 6).

FIG. 16 and FIG. 17 correlate with and are identical with FIG. 7 andFIG. 8.

FIG. 17 shows a schematic cross-sectional diagram of a semiconductorstructure in accordance with a second embodiment of the invention. Thesemiconductor structure is identical with the first embodiment. Thesecond embodiment differs from the first embodiment by using an etchstop layer 18 (FIG. 10 to FIG. 11) when etching a vertical spacer layer16″ in the process of exposing a reduced thickness silicon gateelectrode 16′.

The preferred embodiments of the invention are illustrative of theinvention rather than limiting of the invention. Revisions andmodifications may be made to methods, materials, structures anddimensions in accordance with the preferred embodiments of the inventionwhile still providing an embodiment in accordance with the invention,further in accordance with the accompanying claims.

1. A semiconductor structure comprising: a semiconductor substratehaving a channel region separating a pair of source/drain regions; and agate electrode located over the channel region, where a top surface ofthe gate electrode is no higher than a top surface of the pair ofsource/drain regions.
 2. The semiconductor structure of claim 1 whereinthe top surface of the gate electrode is lower than the top surface ofthe pair of source/drain regions.
 3. The semiconductor structure ofclaim 1 wherein the gate electrode is an element of a planar fieldeffect transistor.
 4. The semiconductor structure of claim 1 furthercomprising a spacer layer laterally adjacent the gate electrode, where aheight of the spacer layer is also no higher than the top surface of thepair of source/drain regions.
 5. The semiconductor structure of claim 1wherein the gate electrode comprises a silicon gate electrode.
 6. Thesemiconductor structure of claim 1 wherein the gate electrode comprisesa silicide gate electrode.
 7. The semiconductor structure of claim 1wherein the gate electrode comprises a metal germanide.
 8. Thesemiconductor structure of claim 1 wherein the pair of source/drainregions comprises a pair of intrinsic source/drain regions and a pair ofraised source/drain layers located thereupon.
 9. The semiconductorstructure of claim 8 wherein the pair of raised source/drain layersimparts a mechanical stress with respect to the pair of intrinsicsource/drain regions.
 10. A method for fabricating a field effecttransistor comprising: forming an extension region into a semiconductorsubstrate which includes at least a gate electrode of a first thicknessthereon, the gate electrode serving as a mask; forming an intrinsicsource/drain region into the semiconductor substrate while using thegate electrode and a horizontal spacer layer formed adjacent thereto asa mask; and etching the gate electrode to a second thickness which isless than the first thickness, thereby forming a reduced thickness gateelectrode.
 11. The method of claim 10 wherein the etching forms areduced thickness silicon gate electrode.
 12. The method of claim 11further comprising the step of reacting the reduced thickness silicongate electrode with a metal silicide forming metal to form a reducedthickness silicide gate electrode.
 13. The method of claim 10 furthercomprising the step of forming a raised source/drain layer upon thesemiconductor substrate while using at least the gate electrode and thespacer layer as a mask.
 14. The method of claim 13 wherein the step offorming the raised source/drain layer upon the semiconductor substrateprovides that a top surface of the raised source/drain layer is higherthan a top surface of the reduced thickness gate electrode.
 15. A methodfor fabricating a field effect transistor comprising: forming anextension region into a semiconductor substrate which includes at leasta gate electrode stack thereon, the gate electrode stack serving as amask, the gate electrode stack comprising a reduced thickness gateelectrode, an etch stop layer thereupon and a vertical spacer layerfurther thereupon; forming an intrinsic source/drain region into thesemiconductor substrate while using the gate electrode stack and ahorizontal spacer layer formed adjacent thereto as a mask; and strippingthe vertical spacer layer from the gate electrode stack while using theetch stop layer as a stop layer, and then further stripping the etchstop layer to expose the reduced thickness gate electrode.
 16. Themethod of claim 15 wherein the stripping exposes a reduced thicknesssilicon gate electrode.
 17. The method of claim 16 further comprisingthe step of reacting the reduced thickness silicon gate electrode with ametal silicide forming metal to form a silicide gate electrode.
 18. Themethod of claim 15 further comprising the step of forming a raisedsource/drain layer upon the semiconductor substrate while using the gateelectrode and the spacer layer as a mask.
 19. The method of claim 18wherein the step of forming the raised source/drain layer upon thesemiconductor substrate provides that a top surface of the raisedsource/drain layer is higher than a top surface of the reduced thicknessgate electrode.
 20. The method of claim 18 further comprising the stepof forming a silicide layer upon the raised source/drain layer.